http://www.girlgeniusonline.com/cgi-...?date=20070625
Agreed, it's unlikely that (defending) flying units can be withheld or hidden from scouts. However it may be possible for ground units to hide under heavy tree cover. Recall way back on page 10 the twoll's comment "I hate ending turn in the open field, y'know? I feel like one tasty target." I suspect what he meant was that, being in the open, he was visible to any possible flyers, whereas if he were in tress he might not be. If that's the case Parson may well have deployed two of his five warlords, along with whatever other ground units he could dwagon-lift in, to reinforce the two 4-dwagon hexes to other side. That could be enough to be the margin of victory should Ansom decide to turn his remaining attack forcd in that direction. Probably not the main trap, but a backup plan.

Next move Parson has some choices, he could launch full scale at Ansom or attack the siege. Which one he attacks is up to Ansom.
H's got at least one more option, possibly the best one of all since it's the one no army should ever leave without adequate cover against a surical air-strike. He can hit Ansom's command center and the dozen or more warlords from the varius factions that were gathered around the plot table. Even if Ansom can get back there himself the archery units he brought with him can't, so he now has to decide where he's going to send what little air cover he has, and whether to pull the remianing archery units off the seige rto cover them. That's if he realizes they're a primary target to begin with, which I believe is precisely Parson's "huge" idea.

If all he hoped to get were the seige units on the next pass, he had that already without setting up the diversion, and given the overall odds he's facing the handful of forestry units that were/can be croaked would have been worth the sacrifice of three dwagons. He's after something much bigger.

The catch is that Parson may have a problem if he's been so caught up in tactical planning that he overlooked the issue of how to get Stanley to go along with whatever plan he comes up with.
Aye, there's the rub. The best laid plans of Hamsters and Men....